Investigations in progress deal with the interaction of the complement system with the cellular immune system. Supernatants of lymphocyte cultures which have been stimulated with C3 and its fragments, C3a, C3b, C3c and C3d, are assayed for a lymphocyte derived chemotactic factor. Although lymphocytes possess receptors for C3, C3b and C3d, only C3 and C3b induce this lymphokine. Studies concerning a modulatory role for C3d are currently in progress.